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Current time:0:00Total duration:11:00

Video transcript

the diode is our first semiconductor device and it's a really important one every other semiconductor is basically made from combinations of diodes and here's a picture of a diode that you can buy this is a just a small little glass package and that distance right there is about four millimeters and inside here right inside here is a little silicon chip and it's manufactured to be a diode so the question is what is a what is a diode a diode is something that conducts current in one direction and does not conduct current in the other direction and the symbol we use for a diode looks like this it has this big arrow here that points in the direction of the forward current that one way to understand how a diode works is to draw an IV curve for it so let's let's draw an IV curve for a diode if it was a perfect diode made in some unknown technology what would happen is in the reverse direction if the voltage across the diode was negative we'll call will label the voltage this way if the voltage across the diode was negative that is this terminal was at a higher voltage than this terminal there would be zero current flowing and then for any positive voltage basically the diode would look like a wire so I can call that that's essentially modeled model number zero of a diode now what we build real diodes what happens is we don't quite get that perfect behavior so in particular if we drew if we build a diode out of silicon we can go to a I'll go to a number one model and a silicon diode actually doesn't conduct to a slight positive voltage and then it would go up like that where this is around 0.6 volts for a lot of simple circuits that we build this is a pretty good IV model of of a diode just as a reminder when we had the IV curve of a resistors a resistor IV curve looks something like this it was a line that went through zero and had a constant slope so ended up so a diode so a diode is a really different kind of device it's a nonlinear device because we can see from this let me move up here and now we'll go to a next level model that is actually the one I want to talk about most this is the model of diode that we use most of the time so I'll call this model number two this is the model that you use when you'll simulate circuits or simulate diodes and we're going to talk about this a little bit more so when you have a diode if I gave you a diode like this and I said what's the IV curve of it so what I would do is I would I would find some sort of box that made voltage for me a power supply with an adjustment on it and then I would also have something that read current so this is an ammeter and this is a voltage supply and we hooked that up like that what we're going to do is we're going to generate this IV curve by making actual measurements of I and V so my first v setting is zero that gives me this point here I hope I measure a current of zero otherwise this thing would be generating power which it's not going to do and then I turn up the voltage slightly and what I notice is there's no current there's no current when it's at point one volts or 0.2 volts and then when it gets to around 0.6 volts on the diode here's VD and here's when the voltage on the diode is around 6 volts what I notice is the current goes up so it goes up to 5 milliamps and then a little bit higher it goes up to 10 milliamps like that and I can plot out all these points along this part of the curve now I go back here and I change the voltage here to read the other way around and that means I'm traveling this way on the on the voltage axis and what I'll read my ammeter will read zero in milliamps 0 0 0 0 to 0 so that plot in this part of the line here now if I make this voltage really large and really negative say I make this like minus 50 volts that's that's this point here what happens is I see a really sharp increasing current like that right there and it keeps going and that is called the breakdown VBR is breakdown and for silicon diodes minus 50 volts is a typical value for that this graph here shows a break in the scale so this is minus 1 volt minus 2 volts and then we go all the way out to 50 volts minus 50 volts and that's where the breakdown occurs and most of the time when we're using diodes we're using them between plus or minus 1 volt across there their terminals so that's how the that's how we know what the IV characteristic of a diode is and what we can do is actually for this section of the curve right here for this part of the curve I can model this with an equation and the equation looks like this this is the IV equation for a diode so this is sort of like the Ohm's law for a diode I equals is this is a current times e to the Q that's the charge on an electron times V on the diode that's the voltage on the diode divided by K t minus 1 K is Boltzmann's constant and T is the temperature of the device measured in Kelvin so this equation actually fits this part of this curve for a real diode it's a it's a fitting curve will look at these constants one at a time is is called the saturation current saturation current and for silicon for silicon that's a value of about 10 to the minus 12 amperes which is 1 Pico ampere that's how much is is Q is the charge on an electron and that equals one point 602 times 10 to the minus 19 coulombs that's Q VD is the voltage across the diode K is Boltzmann's constant that's a small K usually and that equals one point three eight times ten to the minus twenty-three joules per Kelvin and the last variable is T and that's the temperature and that's measured in Kelvin with a big K Kelvin is the absolute temperature scale so zero Kelvin equals minus 273 degrees Celsius very very cold so this right here is the diode equation that's the diode IV equation and it has this exponential shape in it it has its exponential term in it but when we look over here maybe this doesn't look like an exponential curve you haven't seen a curve like that but that actually is just a trick of the scale of this drawing so what I want to do now is I'm going to zoom in really super close right on this origin right here and we're going to see how this exponential term shows up and we'll see what the meaning of is is I equals is times e QV over KT minus one and here's a close up here's an extreme close up on the origin of the diode curve the voltage scale is blown up by about a factor of ten so here's one tenth of a volt forward across the diode and the current scale is super blown up this is in Pico amperes now so this is in ten to the minus twelfth amperes instead of ten to the minus three and you can see here this is a more familiar looking exponential curve and over here there's a little bit of an offset there's a little tiny current in the reverse direction when the voltage is negative and this amount here that's is flowing in the negative direction in the diode if we look at the diode equation and you let V go negative what happens is this term here in the diode equation becomes very very small compared to one and what's left is is is times one and that's what we're looking at right here this is a really small current as you can see from the scale here it's down in the low Pico amps area almost all the time you can ignore this current and just treat it as zero now I want to use a diode in a circuit and we'll see how how we solve circuits that include these nonlinear diodes in them